In a small saucepan, melt butter over medium-high. Stir in flour with a whisk and cook for about 2 minutes. Mix in the thyme. Slowly mix in the soymilk, whisking constantly, and continue cooking until it starts to thicken - about 3 minutes. Stir in the Gorgonzola. Stir until the cheese completely melts in. Season with salt and pepper, remove from heat.

In a 13x9" baking dish lightly coated with nonstick spray, line the bottom with 1/4 of the sliced potatoes. Pour about 3/4 cup sauce over potatoes. Repeat layers twice. Place the remaining potato slices on top. Scatter grated Parmigiano-Reggiano on top.